Electronic paper or e-paper displays address the need for inexpensive yet flexible devices for large area and disposable applications which are unsuitable for standard liquid crystal displays (LCD) and light emitting diode (LED) displays.
Flexible e-paper displays generally use one of the two types of particle displays: suspended particle display (SPD) and electrophoretic image display (EPID). In a SPD, the orientation of the particles is selectively controlled to produce the optical contrast required for a display. In an EPID, the distribution of particle population is selectively controlled in order to produce the optical contrast required for a display. In both cases an electric field is used to control the particles. It should be noted that particles in both display types are suspended in a liquid medium, and in one case the response to the electric field is with respect to orientation, and in the other with respect to distribution.
SPDs are attractive due to their wide viewing angle, high optical transmission and ease of fabrication. In a SPD, light valve action is obtained when sub-micron sized particles with an asymmetric, plate-like shape align with an externally-applied electric field, and thus permit light to pass through (the “light” state). This alignment occurs because the external field induces a dipole moment in the molecules of the particles. In the absence of the external field, the particles orient randomly due to Brownian motion, and consequently block light (the “dark” state). A significant disadvantage of SPDs is that the light areas of the display must be continuously energized with the external electric field to maintain the display, thus consuming energy even when the image on the display is static. SPDs also typically lack a clear voltage threshold (threshold), and require active-matrix addressing for high resolution.
In EPIDs, the particles used in the display are electrically charged and may have a color that contrasts with the liquid used to suspend them. The EPID generally operates by reflection and absorption as opposed to transmission. Although EPIDs have some inherent memory, this memory is due to the viscosity of the liquid medium and therefore decays with time. And because there is no voltage threshold, making multiplexed displays is difficult.
Current e-paper displays have two major problems; volatility (they require continuous power for stable display) and lack of threshold thus making multiplexing difficult. Current solutions for these problems, such as the use of TFT drives, limits the useful size of these displays and dramatically increase their costs. Hence, there is need to solve these and other problems of the prior art.